I agree that Zep and Floyd do have a stronger overall catalog, and shorter as well, but their highest points are pretty much on the same level as these other bands highest points too. DP was pretty much a "1 album a year" band, which I think led to some albums having some filler that shouldn't be there.

Also, maybe another point to why their live albums sold so well too. You could just get all the singles in a single packaging. See: Made In japan